1. Field of the Invention
The present invention relates to an inorganic composition article having superior mechanical strength and capable of reducing the elution of alkali ions from the article by providing a compressive stress layer on the article surface. In particular, with regard to substrates for a magnetic recording medium used in various information magnetic recording devices, and among them, especially to a perpendicular magnetic recording medium, the invention relates to a substrate for a disk-shaped information recording medium having surface super-smoothness, washability and high strength. Herein, in the present invention, “information recording media” refer to information magnetic recording media usable in a fixed mount type hard disk used as a hard disk in personal computers, removable type hard disks or card type hard disks, hard disks for digital video cameras, digital cameras or audio equipment, a hard disk for mobile (cellular) phones or hard disks for various electronic devices.
2. Related Art
In recent years, in order to deal with the application of personal computers to multimedia and also the trend towards handling large amounts of data, such as in animation and speech in digital video cameras and digital cameras, information magnetic recording devices of high capacity have become necessary. As a result, in order to increase surface recording density, information magnetic recording media tend to increase the density of bits and tracks while reducing bit cell size. Therefore, a magnetic head is required to operate more closely to the disk surface.
Furthermore, when the recording density exceeds 100 Gb/in2, the recording becomes thermally unstable in such a small magnetization unit, so that the in-plane recording system reaches a physical limitation with regard to the demand for high recording density exceeding 100 Gb/in2.
In order to cope with this difficulty, the perpendicular magnetic recording system has been employed. Since this perpendicular magnetic recording system has an easy magnetization axis in the perpendicular direction, bit size can be extremely small, and also since this system has a desired medium film thickness (5 to 10 times the in-plane recording system), the demagnetizing field can be reduced and a shape magnetic anisotropic effect can be anticipated. Therefore, this system is able to solve problems such as reduction in recording energy and thermal instability occurring with high density in conventional in-plane recording systems, so as to realize tremendous improvement in recording density compared with the in-plane magnetic recording system. Thus, in the perpendicular magnetic recording system, it has already become possible to achieve a recording density of no less than 100 Gb/in2 on a practical level in mass production, and research on recording densities exceeding 300 Gb/in2 is already being carried out.
In recent information magnetic recording devices, the magnetic head performs CSS (contact•start•stop) motion repeating movements to contact the disk substrate of the information magnetic recording medium before starting the device and float up from the disk substrate when starting a device. In this case, when the contact surface between them is a mirror-finished surface of a more than necessary level, problems occur such as a unsmooth rotation start or damage to an information magnetic recording medium surface caused by an increase in the friction coefficient due to adsorption. Thus, there are conflicting demands regarding the disk substrate of an information magnetic recording medium for keeping the magnetic head floating as low as possible with the increase in the recording capacity, and preventing the magnetic head from sticking to the information magnetic recording medium disk substrate. To cope with these conflicting demands, the development of a landing region technique to produce a starting and stopping unit for the magnetic head in the specific region of the disk substrate of information magnetic recording medium has been promoted.
On the other hand, competing with the aforementioned landing region technique, there has also been developed a ramp loading technique (contact recording of the magnetic head) in which the magnetic head is completely in contact with the substrate so as to remove the starting and stopping of the magnetic head from the information magnetic recording medium, so that unevenness of the substrate surface to prevent the magnetic head from sticking to the disk substrate has become unnecessary. Therefore, by making the substrate surface super-smooth, it has become possible to operate the magnetic head extremely close to the information recording medium surface enabling the bit cell size to be reduced, thereby enabling the recording density to be raised.
Furthermore, also in the perpendicular magnetic recording medium, the magnetic head floating height tends to be as low as 15 nm or less, as the recoding density improves, so that the recording system further tends to become near-contact recording or contact recording. On the other hand, in order to effectively utilize the medium surface as a data region, conventional systems to provide the landing region have been replaced by a ramp loading system with a no landing region. Therefore, the data region of the disk surface or the entire substrate surface must be super-smooth so as to enable the magnetic head floating height to be reduced and enable the contact recording to be performed.
Furthermore, with the improvement of recording density, high precision is needed in positioning the magnetic head and recording medium, so that high dimensional accuracy is required for each component of the disk substrate and magnetic information recording device. Since the effects of the difference of mean linear expansion coefficients on these components becomes significant, it is necessary to make the difference of these mean linear expansion coefficients as small as possible. More strictly, the mean linear expansion coefficient of the disk substrate is often preferably very slightly larger than the mean linear expansion coefficients of these components. Components having a mean linear expansion coefficients in the range of +90˜+100 (×10−7·° C.−1) are often used in small magnetic information recording media in particular, and it has been considered that the thermal expansion coefficient in this range is also necessary for the disk substrate, so that there has been a disadvantage such as the occurrence of a write error when the thermal expansion coefficient deviates slightly.
Since in this perpendicular magnetic recording system, magnetization is performed perpendicularly to the medium surface, a medium having the easy magnetization axis in the perpendicular direction is used that is different from a conventional medium having the easy magnetization axis in the in-plane direction. Materials which have been studied and put to practical use as a recording layer of the perpendicular magnetic recording system are Co base alloys such as CoCrPt, CoCrPt—Si and CoCrPt—SiO2, and an Fe base alloy such as FePt.
However, the film formation temperature for the miniaturization of magnetic substance crystalline particles and production in the perpendicular direction of magnetic recording media such as FePt and other oxide based media have to be raise. Furthermore, according to recent research, annealing may be performed in some cases at high temperature (in the range of 300 to 900° C.) to improve magnetic property. Therefore, the substrate material must be one capable of resisting such high temperatures and should not generate substrate deformation, alteration of the surface roughness, etc.
Furthermore, these magnetic recording medium substrates must have no crystal anisotropy, foreign substances, impurities and so on, which might affect the film forming medium crystal and their composition must be dense, homogeneous and fine, and also must have chemical durability capable of enduring washing and etching with various chemicals.
Furthermore, in order to attempt the expediting of information recording and reading, technical development has been promoted by rotating the information magnetic recording medium disk of the magnetic recording device at high speed. However, the high speed rotation causes deflection and deformation of the substrate, so that the substrate material is required to have high mechanical strength. In addition, in contrast to the current fixed mount type information magnetic recording device, information magnetic recording devices using a removable disk system and card system are in the stage of investigation and practical application, and also since the application development for digital video cameras, digital cameras, and so on has started, these demands have increased more and more.
Although aluminum alloy is mostly used in conventional magnetic disk substrate material, a substrate made of aluminum alloy is apt to produce irregularity in the shape of protrusions or spots on the substrate surface so as to be difficult to obtain a substrate having sufficient flatness and smoothness. In addition, since aluminum alloy is a soft material and apt to be deformed, it is difficult be adapted to the trend to make devices thinner. Furthermore, aluminum alloy has problems such as the occurrence of head clash due to deflection during high speed rotation leading to damage of the medium, so that it is not a material capable of sufficiently adapting to high density recording in future. In addition, since the allowable temperature limit of aluminum alloy during the film formation, which becomes most important in the magnetic recording system, is less than 300° C., the substrate is thermally-deformed if the film formation is performed at higher than 300° C., and annealing is carried out at high temperatures such as 500° C. to 900° C. Therefore, it is difficult to apply a substrate made of aluminum alloy to the substrate for magnetic recording medium requiring such a high temperature treatment.
As a material to resolve these problems of aluminum alloy substrate, there are amorphous glass substrates, chemical reinforced glass substrates, crystallized glass substrates and so on having high hardness and superior impact strength.
As the amorphous glass substrate and chemical reinforced glass substrate, there are known the chemical reinforced soda lime glass (SiO2—CaO—Na2O) and aluminosilicate glass (SiO2—Al2O3—Na2O), but the heat resistance of the substrate itself is low because they are amorphous glasses. That is, there are problems of deformation after the medium film formation where flatness degrades after the film of magnetic recording medium is formed on these substrates at the temperature above 300° C. Furthermore, the above low heat resistance can become the source of problems such as the elution of alkaline components from the inside of the film to cause damage thereto.
While the crystallized glass substrate is superior to other amorphous glass substrates and chemical reinforced glass substrates in Young's modulus and hardness, and concerning the impact strength and ring bending strength, sufficiently fulfills characteristics required as an information magnetic recording device for practical use, it is inferior to an amorphous glass substrate subjected to the chemical reinforcement treatment.
Although the crystallized glass is one in which crystals are precipitated inside by heating amorphous glass of the specific composition and is known to be superior to amorphous glass in its superior mechanical strength, it has been attempted to subject the crystallized glass to the chemical reinforcement treatment so as to further improve its strength. (For example, see Patent documents Japanese Unexamined Patent Application, First Publication No. 2005-37906, Japanese Unexamined Patent Application, First Publication No. Sho61-286245, Japanese Unexamined Patent Application, First Publication No. Hei 05-70174, Japanese Unexamined Patent Application, First Publication No. Sho 49-99521, Japanese Unexamined Patent Application, First Publication No. Sho 59-116150, Japanese Unexamined Patent Application, First Publication No. 2006-62929, Unexamined Patent Application, First Publication No. Hei 10-226532, Unexamined Patent Application, First Publication No. Sho 50-38719 and Unexamined Patent Application, First Publication No. Sho 47-49299, etc.) However, no crystallized glass preferable for application to the magnetic recording medium substrate demanded in recent years has been found.
Although the crystallized glass of Li2O—Al2O3—SiO2 base disclosed in Japanese Unexamined Patent Application, First Publication No. 2005-37906 has β-spodumene (β-Li2Al2Si4O12) as a crystalline phase and its strength is increased by undergoing the chemical reinforcement treatment after precipitating the crystalline phase, it is not preferable as an information recording medium substrate, because its precipitated crystals are material having a negative thermal expansion coefficient. Furthermore, its application is limited to the reflection mirror.
Furthermore, although Na2O—Al2O3—SiO2 base crystallized glass disclosed in Japanese Unexamined Patent Application, First Publication No. Sho61-286245 and Li2O—Al2O3—SiO2 base crystallized glass disclosed in Japanese Unexamined Patent Application, First Publication No. Hei05-70174 have a β-quartz (β-SiO2) solid solution as a crystalline phase, Li2O—Al2O3—SiO2 base crystallized glass disclosed in Japanese Unexamined Patent Application, First Publication No. Sho49-99521 and Japanese Unexamined Patent Application, First Publication No. Sho59-116150 have a β-quartz (β-SiO2) solid solution and/or β-spodumene (β-Li2Al2Si4O12) as the crystalline phase, they are not preferable as the information recording medium substrate, because their precipitated crystals are materials having a negative thermal expansion coefficient. In addition, they have not been discussed for the crystalline particle diameter, Young's modulus and specific gravity.
Furthermore, although Japanese Unexamined Patent Application, First Publication No. 2006-62929 discloses SiO2—Na2O—B2O3—Al2O3 base crystallized glass precipitating crystals of forsterite (2 MgO/SiO2) and/or gahnite (ZnO.Al2O3) as the crystalline phase, its crystal-volume ratio is as high as more than 70%, so that it is difficult to polish the surface roughness after abrasive grinding. In addition, its application is limited to top plates for buildings and cooking utensils.
Although Unexamined Patent Application, First Publication No. Hei10-226532 discloses the chemical reinforcement substrate by ion exchange of SiO2—Li2O—Al2O3 base crystallized glass and Li2O—Al2O3—SiO2 base amorphous glass precipitating at least one kind of lithium disilicate (Li2Si2O5) and/or spodumene (Li2Al2Si4O12), no example concerning the chemical reinforcement of crystallized glass is cited.
In addition, although MgO—Al2O3—SiO2 based crystallized glass disclosed in Unexamined Patent Application, First Publication No. Sho50-38719 has mixed crystals of α-quartz (α-SiO2) and sapphirine (4 MgO.5Al2O3.SiO2) or β-quartz (β-eucryptite solid solution), its crystal amount is as high as 75% by weight, so that it is difficult to polish the surface roughness after grinding. Furthermore, the glasses are not discussed concerning the described below Young's modulus, specific gravity, crystalline particle diameter and crystallization ratio at all.
Unexamined Patent Application, First Publication No. Sho 47-49299 discloses the reinforcement of crystallized glass containing the crystalline phase selected from the group consisting of β-eucryptite (Li2O.Al2O3.2SiO2), β-spodumene (Li2O.Al2O3.4SiO2), nepheline ((Na, K)AlSiO4), carnegieite (Na2O.Al2O3.2SiO2) and β-quartz (β-SiO2), but among the precipitated crystals, those containing β-eucryptite, β-spodumene and β-quartz are materials having a negative thermal expansion coefficient, are not preferable as the information recording medium substrate. Furthermore, when the crystallized glass contains nepheline and carnegieite, it does not include lithium as a component, which can be effective in the chemical reinforcement and further requires a temperature as high as 590 to 730° C. for the chemical reinforcement, so that it is not preferable as a substrate for the information recording medium. In addition, the glasses are not discussed concerning the below described Young's modulus, specific gravity, crystalline particle diameter and crystallization ratio at all.